Acyl desaturase enzymes catalyze the formation of double bonds in fatty acids derived from either dietary sources or de novo synthesis in the liver. Mammals synthesize at least three distinct desaturases, which catalyze desaturation fatty acids at different positions. These enzymes are referred to by the carbon number at which the double bond is inserted, which may occur at the Δ9, Δ6 and Δ5 positions. The resulting mono-unsaturated or polyunsaturated fatty acids are substrates for incorporation into phospholipids, triglycerides, and cholesterol esters.
In human and other mammalian cells, microsomal Δ6 desaturation of the essential fatty acids linoleic acid (18:2n-6) and alpha-linolenic acid (18:3n-3) is an initial and rate limiting step in the biosynthesis of both n-6 and n-3 polyunsaturated fatty acids (PUFAs). The microsomal fraction is the fraction, comprising microspheres and other structures, produced by break-up of the rough endoplasmic reticulum following ultracentrifugation and represents a readily producible form of a number of key cellular enzymes. The products of the aforementioned reactions are gammalinolenic (18:3n-6) and stearidonic acids (18:4n-3) which are subsequently elongated to dihomogammalinolenic acid (DGLA) (20:3n-6) and to the 20:4n-3 product, respectively. The resulting fatty acids are utilized as substrates of a Δ5 desaturase that generates arachidonic acid (20:4n-6) and 20:5n-3. The latter are then further elongated to 22:4n-6 and 22:5n-3, respectively, and finally used to form the products 24:4n-6 and 24:5n-3. The delta-5 desaturase that produces 20:3n9 (or Mead acid) may not be the same enzyme that produces arachidonic acid from DGLA.
In addition to the foregoing, a second microsomal Δ6 desaturation occurs on the PUFAs. The products of this desaturation, 24:5n-6 and 24:6n-3, are converted to 22:5n-6 and 22:6n-3, respectively, by peroxisomal β-oxidation (Sprecher, H. (2000) Biochim. Biophys. Acta. 1486, 219-231). In addition, two sequential elongations of 20:4n-3 (i.e., 22:4n-3 and 24:4n-3) were described by Sauerwald et al. as part of putative alternative steps in the synthesis of 22:6n-3 (Sauerwald, T. U., Hachey, D. L., Jensen, C. L., Chen, H., Anderson, R. E. and Heird W. C. (1997) Pediatr. Res. 41, 183-187). These authors proposed that 24:4n-3 could undergo Δ9 desaturation to 24:5n-3 which can act as substrate of the Δ6 desaturase. Direct elongations on 18:2n-6, 18:3n-3 and 20:3n-6 are also reported.
Evidence shows that the Δ6 desaturase, which recognizes 18-carbon unsaturated fatty acids (18:2n-6 or 18:3n-3), is the same enzyme that desaturates 24-carbon substrates (De Antueno, R. J., Knickle, L C, Smith, H., Elliot, M. L., Allen, S. J., Nwaka, S., and Winther, M. D. (2001) FEBS Letters 509, 77-80, and Innis, S. M., Sprecher, H., Hachey, D., Edmond, J. and Anderson, R. E. (1999) Lipids. 34, 139-149. Cho and coworkers were the first to clone this human Δ6 desaturase and test its activity on the generally used 18-carbon PUFA substrate. (Cho, H. P., Nakamura, M. T. and Clarke, S. D. (1999) J. Biol. Chem. 274, 471-477). In addition, Sauerwald and coworkers (1997) have suggested that two Δ6 desaturation steps active on 18:3n-3 and 18:2n-6 would compete not only with each other but also with 24:5n-3 and 24:4n-6. Sprecher, in a recent review addressing this subject, has emphasized that the control of the Δ6 desaturase would be of considerable interest in animals or human studies if a single enzyme is active on 4 different fatty acids from both n-6 and n-3 families (See: Sprecher, supra).
The delta-6 desaturase carries out desaturation on at least 4 distinct substrates: 18:3n6, 24:4n6, 18:3n3 and 24:5n3. See: De Antueno R J, Knickle L C, Smith H, Elliott M L, Allen S J, Nwaka S, Winther M D , “Activity of human Delta5 and Delta6 desaturases on multiple n-3 and n-6 polyunsaturated fatty acid,” FEBS Lett. Nov 30;509(1):77-80 (2001); D'andrea S, Guillou H, Jan S, Catheline D, Thibault J N, Bouriel M, Rioux V, Legrand P., “The same rat Delta6-desaturase not only acts on 18-but also on 24-carbon fatty acids in very-long-chain polyunsaturated fatty acid biosynthesis,” Biochem J 364(Pt1):49-55 (May 15, 2002). In addition, 18:1 n9 (oleic acid) should also be a substrate (although, perhaps, with lower affinity).
There are several different elongases present in mammalian cells, which are active on different fatty acid substrates. Some of these have been characterized (see Winther et al, WO 02/44320). In at least one example a genetic defect in a human elongase is associated with a human disease, indicating the importance of this enzyme family as a therapeutic target (Zhang, K., Kniazeva, M.,Han, M. et al (2001) Nature Genetics 27, 89-93).
It has been shown (see Brownlie et al, WO 01/62954) that delta-9 desaturase activity may be indirectly measured in a microsomal assay by measuring the release of tritium from the C9 and C10 positions of stearoyl-CoA, in the form of water. In accordance with the present invention, this kind of assay is applicable to a wide range of other desaturase and elongase enzymes using an efficient and economical tritium based assay. Using the process disclosed herein, modulators of fatty acid or triglyceride metabolism are readily identified.